Graphite is commonly used in high temperature applications such as semiconductor processing or metallurgical processes involving molten metals and alloys. Graphite has a melting point of 3550° C., which is the highest amongst the elements, thus with better heat resistance as compared with other materials. The material is readily machinable, possesses good electrical conductivity, provides excellent thermal shock resistance, and has high levels of refractoriness and strength at extremely high temperatures. Also, graphite components are less expensive than similarly sized metallic or ceramic components.
Graphite is frequently used as connection posts for heaters, for holding wafer substrates in a plasma chamber. U.S. Pat. No. 5,343,022 discloses such an application, wherein graphite posts are used to connect the heating element of the heater to an external powder supply. U.S. Patent Publication No. 2004173161 discloses a wafer support assembly with a graphite concentric post (either solid or hollow) for connecting the heater to an external power supply. U.S. Pat. No. 5,233,163 discloses a heater having a columnar body and a pair of graphite “legs” or connection posts.
In one embodiment of the prior art as disclosed in U.S. Pat. No. 5,343,022 and U.S. Patent Publication No. 2004173161, the graphite posts are coated with at least a protective layer, e.g., boron nitride, aluminium nitride, and the like, protecting the graphite from the reactive environment. However, certain parts of the graphite posts must be exposed so that electrical connection can be made to the graphite post and hence the heater. The coating layer provides protection to graphite parts in corrosive environment. However, graphite that is suitable for accepting coating typically has poor mechanical strength, with tensile and flexural strength ranging from 1-4 ksi. The typical values of tensile and flexural strength of metals used for such connections, e.g., tantalum and molybdenum, are 100 ksi. The poor mechanical strength of graphite can lead to cracking or breaking of the graphite part, i.e., the graphite post, when a load or stress is applied. This can be during the connection of the graphite post to the external power supply, during thermal cycling, during installation into the process tool, and sometimes even during shipping. The failure part is quite often than not, initiated in the area that is not encapsulated with a protective coating layer. Coated graphite sections have quite often failed in operation as well, although not as frequent as with the uncoated part. Either failure mode, in the coated section or uncoated section of the graphite part, will render the heating unit non-functional.
There is a need for to extend the life of and strengthen graphite parts, particularly as connection posts for heaters, without compromising the inherent properties of graphite such as excellent electrical conductivity and thermal shock resistance.